Increasingly, synchronous rectifiers are replacing freewheeling diodes in non-isolated DC-DC buck converters in order to increase the power conversion efficiency of the converters. One feature of non-isolated DC-DC converters with synchronous rectification is that current is enabled to flow not only to the output terminals through the synchronous rectifier but also in a reverse direction from the output terminals back into the converter, i.e., a non-isolated dc-dc converter with synchronous rectification can have both current-sourcing and current-sinking capability.
A conventional buck converter is shown in FIG. 1. As is well known, a basic buck converter comprises a switch 6, an input filter capacitor 8, a freewheeling diode 12, an inductor 14, and a capacitor 16, connected in a conventional way between an input terminal 2 to which is coupled an input voltage Vin relative to ground, and an output terminal 22 at which the buck converter generates a regulated output voltage Vo relative to ground. An exemplary load 20 is shown coupled to the output of converter 10. The switch 6 is typically an electronic switch, such as a MOSFET, that is controlled in a known manner by a control circuit, e.g., a pulse width modulator (PWM) (not shown in FIG. 1) that is responsive to the output voltage Vo. When the switch 6 is closed, the capacitor 16 is charged via switch 6 and inductor 14 from the input voltage Vin to produce the output voltage Vo, which is consequently less than the peak input voltage Vin. When switch 6 is open, current through the inductor 14, identified as Io, is maintained via diode 12.
In order to boost power conversion efficiency, the freewheeling diode 12 is preferably replaced with a MOSFET, defined as a synchronous rectifier, identified as 18 in FIG. 1 and shown connected using dotted lines. In operation, synchronous rectifier 18 lowers the voltage drop across nodes 7 and 5 that otherwise exists with diode 12. Only uni-directional current flow is permitted through the freewheeling diode 12. By contrast, the synchronous rectifier 18 permits bi-directional current flow. As a result, inductor current, Io, can flow in reverse through synchronous rectifier 18 from the output. Synchronous rectifier 18 is preferably controlled directly by a PWM (not shown). Although switch 6 and synchronous rectifier 18 are both driven by a PWM, it is well known that the control signals from the PWM for these elements are complementary signals such that switch 6 and synchronous rectifier 18 are never turned on at the same time, in order to prevent the shorting of the input terminal 2 to ground.
The bi-directional current flowing capability of the synchronous rectifier 18 may pose a serious problem when such rectifiers are used in paralleled power converters. The paralleling of power converters provides a way for two or more individual, small, high density power converter modules to supply the higher power required by current generation loads and/or to provide redundancy. Applications may also require various configurations of paralleled converters. A known application, e.g., for a digital signal processor, requires paralleled converters to be configured for sequential operation, wherein the converters are powered on sequentially according to a predetermined sequence. FIG. 2 is a block diagram of a prior art system having two paralleled power modules connected in a sequencing configuration to supply power to two loads. The parallel sequencing system 30 in FIG. 2 includes a converter 32 connected in parallel with a converter 34. According to the sequencing for an embodiment of system 30, converter 32 is always turned on before converter 34 is turned on. Each converter 32, 34 is a buck converter having a synchronous rectifier in place of the freewheeling diode, as shown in FIG. 1. As shown in FIG. 2, power is supplied to converters 32, 34 from a single power input, Vin, at input terminals 2, 4. It will be recognized by those skilled in the art that it is not necessary that power be supplied to the converter at a single power input port. Rather, each power module may receive power from a separate power source such as separate AC-DC converters (not shown). Converter 32 is coupled to output terminals 42 and 44 to supply an output voltage VA0 to a load, shown schematically as 28. Converter 34 is coupled to output terminals 38, 40 to supply an output voltage VB0 to a load, shown schematically as 26. The output of each converter 32, 34 is also coupled to the output terminals of the other converter via a diode 36. Diode 36 has an anode coupled to output terminal 42 of converter 32 and a cathode coupled to the output terminal 38 of converter 34. The corresponding negative output terminals 44, 40 of each converter are also connected as shown in FIG. 2.
In operation, converter 32 is turned on first while converter 34 remains off. During this time, the synchronous rectifier in converter 34 remains in an off state. At this time, converter 32 supplies an output voltage VA0 to a load 28. However, since converter 34 is off, diode 36 is in a conduction state. As a result, converter 32 also provides power to a load 26. At this point in the sequence, converter 34 is turned on. As converter 34 begins to operate, its synchronous rectifier, now turned on, will pull down the paralleled outputs to a level corresponding to the programmed soft-start level for converter 34. This pulling-down effect causes a short circuit operation of converter 32 during the soft-start period for converter 34. This effect is one example of an effect commonly referred to as the “synchronous rectifier back bias” problem of non-isolated dc-dc buck converters. The synchronous rectifier of converter 34 will continue this “pulling-down” effect until the output voltage of converter 34 becomes equal to the output voltage of converter 32, at which point diode 36 no longer conducts and the two converter outputs become uncoupled from one another. In practice, a short circuit protection will be triggered and the system 30 cannot remain in operation without special attention. A need therefore exists for overcoming this synchronous rectifier back bias problem for the system of FIG. 2, while having the benefits provided by the use of a synchronous rectifier, namely reduced cost and higher density, as demanded for modern devices.
FIG. 3 is a block diagram of another configuration of a system of parallel converters (also referred to herein as “power modules”). For the paralleled converter configuration shown in FIG. 3, power is supplied to a common output voltage bus and thereby to a load. As shown in FIG. 3, power module 1, power module 2, . . . power module N are each coupled to a single power output port 320 for supplying power to a load. An exemplary load 330 is shown coupled to output port 320 of system 300. In a preferred embodiment, power is supplied to power modules 1 through N at a single power input port 340. It will be recognized by those skilled in the art that it is not necessary that power be supplied to power modules 1 through N at a single power input port. Rather, each power module may receive power from a separate power source such as separate AC-DC converters (not shown).
In one exemplary system, the power modules 1 through N are buck converters having a synchronous rectifier in place of the freewheeling diode, as shown in FIG. 1. For this exemplary system, because the synchronous rectifier allows reverse current flow, a system failure may result, e.g. from recycling one or more modules while the system is already in operation, and powering on each of paralleled modules at different times, etc.
A need therefore exists for a circuit that actively and efficiently controls the synchronous rectifier in the respective power converters in a system having paralleled power converters in order to eliminate the synchronous rectifier back bias problem. There is also a need for a circuit that provides this function during the soft start period of a power converter in a paralleled converter configuration.